In blow molding, it is normal to press fit a blow core die in a neck portion of a preform which is disposed in a blow cavity mold and to introduce blow air with the neck portion airtightly sealed.
As a different technique from this, blow nozzles are proposed which airtightly seals the neck portion without press fitting the blow core die in the neck portion (Patent Documents 1 to 3). According to these blow nozzles, even with a preform which is reduced in weight and which includes a neck portion whose mechanical strength cannot be ensured sufficiently due to being thinned, deformation of the neck portion can be prevented.